1. Field of the Invention
The present invention relates to a phase conjugate interferometer for high precision shape testing of paraboloidal mirror surfaces, and more particularly to a phase conjugate interferometer for high precision testing of the shape of paraboloidal mirror surfaces, using a Fizeau type interferometer with an optical fiber laser beam guide.
2. Description of the Prior Art
The ability of paraboloidal mirror surfaces to precisely converge a parallel beam of light to a single point make them extremely useful as converging reflectors for laser processing, where they permit precision processing, and as primary mirrors in telescopes, where they make it possible to realize detailed observations.
Previously the technical difficulty of fabricating paraboloidal mirror surfaces has made them costly and limited their use. In recent years, however, advances in very high precision processing technology and the use of diamond-tool machining and the like have made it possible to fabricate paraboloidal reflectors relatively simply and at low cost, and this is causing them to be used more widely.
When a paraboloidal mirror surface is fabricated, the mirror surface finish has to be tested. This testing is usually done with an interferometer, which does not involve contact with the surface being measured, provides high precision and enables the entire surface to be tested simultaneously. However, a major problem with a Fizeau type high precision interferometer that it uses a double reflection system and it is impossible to identify the position of surface shape errors.
This is explained with reference to FIG. 2, which shows a Fizeau type high precision interferometer. A laser beam from a laser light source 1 is diverged by an objective lens 3 after focussing at pinhole 2, transmitted by a half mirror 4 and collimated by a collimating lens 5 and impinges on a paraboloidal reflector 6 being tested. An auxiliary flat reflector 7 reflects the light reflected by the paraboloidal reflector 6 back onto the paraboloidal reflector 6, and the light is then projected onto a screen 9 by the half mirror 4. Assuming the paraboloidal reflector 6 has a shape error portion 6', when the light reflected by the paraboloidal reflector 6 is reflected back to the paraboloidal reflector 6 by the auxiliary flat reflector 7, light reflecting from the shape error portion 6' will impinge on point 8. It has been found that when the shape error portion 6' has a large inclination it becomes impossible to identify that the error is at portion 6'.
To overcome this drawback, the single reflection system Mach-Zehnder high precision interferometer was designed as an improvement over the double reflection Fizeau type interferometer. As shown by the Mach-Zehnder interferometer arrangement shown in FIG. 3, a laser beam from a laser light source 10 is diverged by a lens 11, passed through collimating lens 12 and impinges on a semi-transparent mirror 13. Light reflected by the semi-transparent mirror 13 passes via mirrors 14, 15 and 16, semi-transparent mirror 17 and lens 18, whereby it is projected onto a screen 19. The light that is transmitted by the semi-transparent mirror 13 passes through collimating lens 20 and impinges on paraboloidal reflector 21, from which the light is reflected through collimating lens 22 and onto the semi-transparent mirror 17. The light that is reflected by the semi-transparent mirror 17 is projected onto the screen 19 via the lens 18.
However, the above interferometer requires numerous high precision lenses, which are difficult to adjust and make the system costly. Moreover, because the light travels by different paths, the system is prone to the effects of air turbulence and external vibration, which can degrade the precision of test results, in addition to which the apparatus is large and bulky.
The object of the present invention is to provide an improved, high precision phase conjugate interferometer that is compact is low in cost and has an optical system that can be readily adjusted.